In a power steering device of which steering motion is assisted by rotational power transmitted from a motor to steering mechanism therein, wherein the rotational power is generated by the motor responding to rotational manipulation applied over steering elements such as a steering handle, it is necessary for driving control of the steering assist motor to detect steering torque applied over the steering elements. For this detection demand, a torque sensor has conventionally been used at an intermediary position on the steering shaft that connects the steering elements and the steering mechanism.
In an electrically-assisted power steering device that uses a torque sensor, detection of the heading of steering control wheel (hereinafter referred to as steering angle) that varies according to manipulation of the steering handle, as well as the steering torque, is required for application of the steering angle to various control scheme of a vehicle.
Conventional arts have employed a mechanical rotational angle sensor for detection of the steering angle. For example, Patent Literature 1 has described a mechanical rotational angle sensor comprised of a pair of gears having different numbers of teeth each from the other, which freely rotate around an axis parallel to the steering shaft being interlocked with the turn of the steering shaft. This sensor detects the steering angle based on the data obtained by means of combining the rotation angles of each of gears of the pair sensed with a magnetic sensor.
In addition to such a mechanical sensor as is described in Patent Literature 1, an index sensor is listed as a means for detecting the number of turns of the steering shaft. The index sensor is a device that generates a reference position of rotation as the steering shaft turns.
The fundamental configuration of an index sensor will be explained referring to FIG. 5. FIG. 5 is a cross-sectional view of an index sensor installed on a steering shaft (the shaft of a steering handle) viewed in the axial direction of the steering shaft. In the index sensor 21 shown in FIG. 5, a yoke 23 made of a material having high magnetic permeability is installed on a steering shaft 4 sandwiching a collar 22 underneath. Outside of the collar 22, a magnet 211, a Hall element 212 (a magnetic sensor) that detects variation of density of magnetic flux generated from the magnet 211, and a comparator 213 that outputs reference position signal when the detected variation of density of magnetic flux becomes in excess of a predetermined threshold are arranged in line in the direction from the rotational center of the steering shaft 4 toward radially far-side.
The function of the index sensor will be explained referring to FIG. 6. In FIG. 6, such a state that the steering manipulation is not applied, i.e. the steering shaft 4 is at the reference position is shown in FIG. 6(b) and such a state that the steering shaft 4 is being turned by the steering manipulation is shown in FIG. 6(a). In the state shown in FIG. 6(b), the yoke 23 is at the position closest to the magnet 211. Since the yoke 23 is made of material having high magnetic permeability, magnetic flux 26 generated from the magnet 211 concentrates into the yoke 23 when its position is the above-stated reference position causing the magnetic flux density that the Hall element 212 will detect to be the maximum.
When the steering manipulation is applied causing the steering shaft 4 to turn, the yoke 23 shifts from the magnetic axis of the magnet 211 as shown in FIG. 6(a) with the magnetic flux 26 concentrating on the yoke 23 directed toward the opposite pole of the magnet 211 (S-pole in the figure) resulting in the Hall element 212 detecting reduced magnetic flux density. When the amount of shift of the rotational angle from the reference position becomes large to a certain extent, the flux density that the Hall element 212 detects reduces down to a certain level since the flux 26 concentrating the yoke 23 reduces.
The steering manipulation for changing the wheel traveling direction generally would not end in one turn but will end after plural turns to reach its maximum steering angle. During the course of this steering manipulation process, the magnetic flux 26 generated from the magnet 211 concentrates into the yoke 23 causing the magnetic flux density that the Hall element 212 will detect to be the maximum whenever the steering shaft 4 passes the reference position. As FIG. 5 shows, the index sensor 21 is equipped with the comparator 213 that outputs reference position signal when the detected variation of density of magnetic flux becomes in excess of a predetermined threshold. Therefore, the reference position signal is generated from the comparator 213 whenever the steering shaft 4 passes the reference position causing the magnetic flux density that the Hall element 212 will detect to exceed the predetermined threshold. That is; the number of turns of the steering shaft 4 can be detected by sensing the pass of the yoke 23 over the reference position by the index sensor 21 and by counting the number of passes over the reference position, wherein the yoke 23 is installed on the steering shaft 4 sandwiching the collar 22 underneath.
Usually, a power steering device is equipped with a motor for steering assist. Although rotation angle of this motor provides steering angle information, the information by the rotation angle of this motor is limited to the range of 0° to 360°. This means that the obtained information is relative value of angles; the absolute value of steering angle cannot be obtained from that. However, combining the information of number of turns of the steering shaft obtained by the index sensor with the information of relative value of angles obtained form the steering assist motor makes it possible to accurately find the absolute value of the steering angle caused by the steering manipulation.
Thus, the use of an index sensor enables to detect the steering angle without using such a mechanical rotational angle sensor as described in Patent Literature 1. This feature offers advantages such as reduction of number of constituent parts, simplification of fabrication process, cutting of manufacturing cost, and miniaturization of device.
{Patent Literature 1}
    Japanese Patent Application Laid-Open No. 2007-269281